You wont ever need a .21 in a st. It will just rip the drive train to shreds. People have problems with a .18 cvr tearin up drive train parts, let alone a .21. The .18cvr has all the power that you will ever want in a st. It already spins the tires to no end and will do wheelies on demand if you can get the tires to hook up. A .18 is all you will ever need.